Scintillation counter, segmented shield

ABSTRACT

A scintillation counter, particularly for counting gamma ray photons, includes a massive lead radiation shield surrounding a sample-receiving zone. The shield is disassembleable into a plurality of segments to allow facile installation and removal of a photomultiplier tube assembly, the segments being so constructed as to prevent straight-line access of external radiation through the shield into radiation-responsive areas. Provisions are made for accurately aligning the photomultiplier tube with respect to one or more sample-transmitting bores extending through the shield to the sample receiving zone. A sample elevator, used in transporting samples into the zone, is designed to provide a maximum gamma-receiving aspect to maximize the gamma detecting efficiency.

United States Patent Olson et al.

[4 1 May 13, 1975 SCINTILLATION COUNTER, SEGMENTED SHIELD Inventors:Robert E. Olson, Glen Ellyn; Arnold David Thumim, Chicago, both of ill.

[73] Assignee: Packard Instrument Company, Inc.,

Downers Grove, Ill.

[22] Filed: July 27, 1973 [21] Appl. No.: 383,097

[57] ABSTRACT A scintillation counter, particularly for counting gammaray photons, includes a massive lead radiation shield surrounding asample-receiving zone. The shield is disassembleable into a plurality ofsegments to allow facile installation and removal of a photomultipliertube assembly, the segments being so constructed as 2? 'i g lg toprevent straight-line access of external radiation f 505 506 through theshield into radiation-responsive areas. 1 0 b Provisions are made foraccurately aligning the photomultiplier tube with respect to one or moresampletransmitting bores extending through the shield to the [56]References cued sample receiving zone. A sample elevator, used in UNITEDSTATES PATENTS transporting samples into the zone, is designed to pro-3,198,948 8/1965 Olson 250/328 vide a maximum gamma-receiving aspect tomaximize 3,432,666 3/l969 Nash et al 25U/506 the gamma detectingefficiency,

5 Claims, 12 Drawing Figures I Z )1? f f j .1/ 4' 1" a 4% 4 J; E I

-- W id \cz .RQE a PAIENTED HAY I 3 i975 SHEU 2 BF 4 SCINTILLATIONCOUNTER, SEGMENTED SHIELD CROSS REFERENCE TO RELATED APPLICATION Threeapplications. filed simultaneously. are directed to features of thepresent invention:

L. Robert E. Olsen application Ser. No. 3810M, filed July 27,1973entitled Scintillation Counter; Photomultiplier Tube Alignment."

2. Arnold David Thumim application Ser. No. 388,085, filed July 27, 1973entitled Scintillation Counter; Maximum Gamma Aspect.

3. Frank application Ser. No. 241,987, filed Apr. 7, 1972 and nowabandoned entitled Elevator Mechanism for Scintillation Detectors andthe Like.

BACKGROUND OF THE INVENTION This invention relates to apparatus for themeasurement of radioactivity, and more particularly concerns improvedscintillation counting equipment.

The detection and measurement of radioactivity by the use ofscintillation counters is conducted routinely in many laboratories.Scintillation counting equipment is commercially available, and iscapable of unusual precision and accuracy. Nonetheless, the fact thatmany radionuclide samples are of low activity presents serious problemsin the practical conduct of scintilla tion counting.

One of the earliest-recognized problems is background radiation. whichoccurs chiefly from cosmic sources, but may present as a result ofnormal radioactivity of many natural elements, from luminous wristwatchdials, and the like. This background radioactivity, in the case ofsamples which themselves have little radioactivity, must be excluded tothe fullest extent possible from the sample counting zone.

Conventionally, the sample counting or sample receiving zone is encasedwithin a massive lead radiation shield, which also encases aphotomultiplier tube assembly, that is, an electron multiplier tubehaving a photoelectic cathode which is capable of responding to very lowlight levels. ldeally, the shield excludes all ex ternal radiation, sothat the photomultiplier tube will respond only to light scintillationsproduced by interaction of radiation from the sample with a scintillatormaterial present in or near the sample receiving zone.

However, because the shield is usually of massive thickness, it isextremely heavy; shields of several hundred pounds are not uncommon. Yetthe shield must include provisions for the installation and removal ofthe photomultiplier tube. However, the parting surface of adjacentshield segments provides an essentially unobstructed route for externalradiation to enter the sample receiving zone or other portions of thecounter where they could produce a false count in the event the segmentsdo not match perfectly. Perfect matching of heavy lead components isvirtually impossible.

Accordingly. one object ofthe invention is to provide a radiation shieldfor scintillation counters, which shield is capable of disassembly intoseveral segments that, when assembled, offer no straight-line access forexternal radiation to enter the sample receiving zone or otherradiation-responsive areas. Another object is to provide such shieldassembly where the segments are easily removable to allow installationand removal of photomultiplier tubes. Still another object is to providea segmented lead radiation shield which is readily fabricated and iscapable of repeated assembly and disas- 2 sembly without impairing thebackground-isolating function of the shield.

An additional shield design problem arises from the fact that manyscintillation counters must function automatically. with automaticintroduction of a sample into the sample receiving zone and forautomatic with drawal of the sample at the end of a counting period. Toavoid both mechanical jamming of the transporting equipment and toprevent the inducement of spurious light flashes as a result of a samplecontacting the walls ofthe scintillation counter, it is essential thatthe transporting equipment and the photomultiplier tube assembly beaccurately aligned with respect to each other. Because the sampletransporting equipment and the photomultiplier tube assembly areessentially inaccessible once in the radiation shield, and because theproblems caused by misalignment are so serious, it is impor tant toprovide an absolute system for aligning these components.

Accordingly, an additional object is to provide a scintillation counterapparatus in which the photomultiplier tube assembly is alignable bothlongitudinally and perpendicularly with respect to the sampletransporting equipment. A further object is to provide an automaticscintillation counter which is free of the problems associated withmisaligned sample transmitting elevators and the like.

Further, it is also desirable to maximize the aspect of thephotomultiplier tube or tubes; in other words, to permit the tube toobserve as much as possible of the sample. Radiation emitted from thesample which is prevented from access to a scintillator material, orscintillations which are prevented from access to a photomultipliertube, inevitably produce a reduction in the detecting efficiency of thescintillation counter. Still another object of the invention is toimprove the gamma ray detecting effeciency of scintillation spectrometerby maximizing the transmission of gamma rays from a gamma-active sampleto a scintillator crystal.

SUMMARY OF INVENTION The utility and versatility of scintillationcounters are significantly improved in several respects.

First, the massive lead radiation shield is made in a plurality ofsegments which are disassembleable to allow installation and removal ofa photomultiplier tube assembly. To prevent straight-line access ofexter nal radiation through the lead shield into the samplereceivingzone or into the scintillator(s) and photomultiplierls), at least threemajor segments are employed; a sample-Zonesurrounding segment having anaxial cylindrical bore sufficiently large in diameter to receive aconventional photomultiplier tube assembly; an intermediate segmenthaving an axial cylindrical bore coaxial with the first segment, theintermediate segment being longitudinally divided into two segmentsadapted for radial outward movement; and an end cap segment covering thebore of the intermediate segment.

ln further keeping with this feature of the invention, thesample-zone-surrounding segment desirably has a protruding collar on itsparting face, while the intermediate segment has a mating recess on itsparting face. Also, the intermediate segment is advantageously dividedby a pair of parallel parting surfaces, each tangential to thecylindrical bore of the intermediate segment.

Second, to render the photomultiplier tube assembly alignable bothlongitudinally and perpendicularly with respect to either asample-conducting (sample transferring] bore or a sample elevator bore,or both, a tube is slidably positioned in the bore or bores which has aflat end facing the sample-receiving zone. The photomultiplier tubeassembly, which is normally rotatable as well as longitudinally movablein a bore within the radiation shield, is provided with a flat portionand a step portion on either (or both) the top or bottom of theassembly. By registering the tube with the flat portion and stopportion, alignment of the photomultiplier tube assembly with respect tothe bore or bores is as sured.

Third, to obtain maximum aspect for gamma detection, the samplereceiving zone is made substantially longer than the sample, and asample elevator exten sion is provided which has a plurality of thinvertical sample-supporting members. The extension platform thuspositions the sample above the bottom of the sample receiving zone, sothat transverselycmitted gamma radiation leaving the bottom of thesample is permitted to reach an adjacent scintillator crystal.

BRIEF DESCRIPTION OF THE DRAWINGS Other objects and advantages of theinvention will become apparent from the following detailed descriptionand upon reference to the drawings, in which:

FIG. I is a front elevation view showing an assembled shield or pig forcontaining a sample being simultaneously observed by two photomultipliertubes;

FIG. 2 is atop view of the shield depicted in FIG. I, the view beingtaken along plane 2-2 of FIG. 1;

FIG. 3 is a sectional view, taken along plane 33 of FIG. 2, depictingthe internal construction of the scin tillation counter shield assembly;

FIG. 4 is a sectional view, taken along plane 44 of the radiationshield, and also illustrating, in phantom. the horizontal displacementof components of the segments to provide access to the photomultipliertube assembly',

FIG. 5 is a perspective of an elevator extension platform employed tomaximize the detecting efficiency of the scintillation counter;

FIG. 6 is a sectional view, taken along plane 66 of FIG. 5, of theelevator;

FIG. 7 is a sectional view of the elevator of FIG. 5, taken along plane77 of FIG. 6',

FIGS is a front sectional view of a scintillation crystal assemblyincluding provisions for aligning a photomultiplier tube assembly,including a sample counting Zone, with sample transporting assemblies;

FIG. 9 is a top partial enlarged view of the scintillation crystalassembly of FIG. 8, taken along plane 9-9 of FIG. 8;

FIG. 10 is a partial sectional view of the scintillation crystalassembly of FIG. 8, taken along plane I0I0 of FIG. 9;

FIG. II is a partial front sectional view of a sample counting zone,with sample and elevator in place, for counting gamma ray photons underconditions maximizing the gamma detecting efficiency of the scintillation counter, and

FIG. 12 is an exploded view ofthe radiation shield of FIGS. 1-4, andshowing the arrangement of the elevator extension platform of FIGS. 5-7and the scintillation crystal assembly of FIGS. 8-1 I.

While the invention will be described in connection with a preferredembodiment, the reader will understand that it is not intended to limitthe invention to that embodiment or to the specific combination withother components. On the contrary. it is intended to cover allalternatives, modifications, and equivalents as may be included withinthe spirit and scope of the invention as defined by the appended claims.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT The radiation shieldassembly of a scintillation counter particularly suited for gammadetection is depicted in FIGS. 1 through 4 and, in an exploded view, inFIG. 12. In the particular shield assembly shown, two photomultipliertube assemblies, including two crystals, are employed to increaseresolution and efficiency of a flow through crystal design.

As shown in these figures, the lead radiation shield, indicatedgenerally at 11, is of generally cylindrical overall shape. The shieldII includes a central cylindrical segment 13, a pair ofintermediatessegments 14, IS on opposite ends of the central segment 13, and a pairof end cap segments l6, 17 at the ends of the intermediate segments l4,15 respectively. An axial cylindrical bore 19 (FIGS. 3, 12) extendsthrough the segment l3, and is coaxial with similar cylindrical bores20, 21 (FIGS. 3, 12) through the intermediate segments 14, I5,respectively, to receive a pair of photomultiplier tube assemblies 23,24 (FIGS. 3, I2), to be described presently.

As best shown in FIGS. 4 and 12, the intermediate segments I4, 15 areeach made in two parts. The longitudinal division or parting planes 26which define the parting surfaces between the segments 14a, 14b, are soshaped as to obstruct the passage of nuclear radiation, which travelsalong a straight line, from entering a sample receiving zone 28 (FIGS.3, II) where a sample is placed for scintillation counting or fromcontacting the scintillator crystals and other components of thisphotomultiplier assembly.

Various surfaces are available for avoiding straightline transmission,but the most effective and yet simplest is obtained by making theparting planes 26 in the form of two parallel planes 26a, 26b, eachparallel to axis of the bore 20, but tangent to this bore. Consequently,radiation which could travel between the parting surfaces has noopportunity to enter the sample receiving zone 28 by reason of geometricrelationship between the tangential lines 26a, 26b, and the bore intowhich the planes extend.

By the same token, dividing the intermediate segment I4 into twohalf-segments 14a, 14b, permits the halves to be moved radially andhorizontally away from the assembled shield 13, as shown in FIG. 4.Rollers 30, 3I (FIG. 1) permit the segments I40, 14b, to be rolled outof the way along horizontal paths without disturb ing the centralsegment 13 which surrounds the sample receiving zone 28.

To prevent straight-line access of radiation through the parting planes33, 34 between the central segment 13 and the intermediate segments 14,I5, one of the surfaces is provided with a protruding collar 3l whichmates with a generally conforming recess 32 on the opposite segment.Advantageously the collar 3I is on the sample-zone-surrounding segment13, as shown in FIGS. 3, 12, to permit disassembly of the two portionsof the intermediate segment merely by radial displacement withoutrequiring any axial displacement as would be the case if the protrudingcollar 31 were on the intermediate segments l4, 15.

At the ends of the shield II, a pair of end caps I6, 17 (FIGS. I, 2, 3,I2) are employed to terminate the shield assembly. These caps 16, I7 aregenerally flat cylindrical discs, approximately as thick as the wallthick ness of the segments l3, l4, 15. To permit external electricalcommunication with the photomultiplier tube assemblies 23, 24, thesegments I4, I4uhave a groove positioned within a plane such that anyradioac tivity passing through this opening will not cause any increasein background. These are omitted from the drawings.

As an additional obstruction to cosmic radiation, a rectangular leadplate 34 is mounted above the samplezone-surrounding segment I3 and issupported on a set of brackets 35, 36. This plate is ordinarily notremoved from the central segment 13 during either assembly or use of theshield 11.

All segments of the shield assembly II are held together with a seriesof four hooks 38, threaded at their straight ends, and corresponding eyebolts 39 which abut against tie bars 40. The hooked portions removablyengage a pair of horizontal bosses 42 (FIGS. 2, I2), permitting theintermediate segments 14, I5 and the end cap segments l6, 17 to beremovable while the central segment 13 remains fixed in place on itsweldment base 43 (FIGS. 1, 3). Handles 44 on each of the intermediatesegments 14a, 14b, 15a, 15b and on the end cap segments 16, I7 (FIGS. I,2, 3, I2) permit either manual or hoist-operated vertical movement ofthese respective segments to facilitate (in the case of the intermediatesegments) radial outward movement and (in the case of the end capsegments l6, l7) axial outward movement along their respective rollers30, 3I and support plates 46, 47 (FIG. I).

In the shield assembly II depicted in the drawings, two photomultipliertube assemblies 23, 24 (FIG. 3) are used. However, for many applicationsonly a single photomultiplier tube assembly is required, and in thisevent the shield assembly may be simplified. For example, only one setof intermediate segments 14a, 14!), or 15a, 15b is required, as is onlyone end cap 16 or 17. However, the central segment I3 in this case has abore 19 extending only part way through the central segment, and the endof the segment opposite the bore serves to exclude radiation from theshield end opposite that ofthe end cap. In other words, the centralsegment 13, instead of being a hollow cylinder as shown in FIG. 3, is acup-shaped structure with the bore 19 extending from one side to onlyjust beyond the samplereceiving zone.

As indicated in FIG. 3, and partially in FIGS. 1 and 12, samples aretransmitted or conducted into the sample receiving zone 28 (FIG II)within the shield ll via a sample-conducting bore 50 extendingvertically upward from the zone 28 through the central segment I3 andthe plate 34. A similar bore SI (FIGS. 3, II) ex tends from the samplereceiving zone 28 through the bottom of the central segment 13 toreceive a sample conducting elevator assembly. This assembly, which isdescribed more fully in Frank application Ser. No. 241,987, filed Apr.7, 1972 and entitled Elevator Mechanism for Scintillation Detectors andthe Like," includes a flexible elevator band 52 (FIGS. 1,3, 5, 11,

12) passing through a guide 53 to an elevator drive, not shown, whichraises or lowers the band 52 to raise or lower a sample bottle 55 (FIG.11) into the sample receiving zone 28.

As best shown in FIGS. 3 and II, the bores 50, 5] may include thin buthard metal tubular bushings 56, 57 to prevent wear on the tubes 50, SIand, optimally, also have a dual'functioning pair of a sampleconductingtube 58 and an elevator tube 59. These latter tubes are slidable in thesample-conducting bore 50 and the elevator bore SI, respectively, andnot only insure against bore wear throughout numerous sampleload-count-unload cycles, but serve to align the photomultiplier tubeassemblies 23, 24 (FIG. 3). In common with most photomultiplier tubeassemblies. the assemblies 23, 24 shown in FIG. 3, each includes aphotomultiplier tube such as the tube 60 having a frustro conicalportion 6l with its broad portion 63 facing toward a scintillatorcrystal 64 and toward the sample receiving zone 28, the narrower face ofthe frustro conical portion facing away from the sample receiving zone28 and toward a photomultiplier tube socket 66. Penetrating radiationfrom a sample (55 in FIG. 11) interacts with the material in thescintillator crystal 64, where it is converted to light flashes whichare detected by the photomultiplier tube 60 and converted intoelectrical signals transmitted via the photomultiplier tube socket 66.

Photomultiplier tube assemblies of this type, how ever, are difficult toposition and align with respect to a sample conducting bore 50 and/orwith respect to an elevator bore SI. To accommodate slightly differentsizes of tube assemblies 23, 24, which occur normally as manufacturingtolerances, the tube assemblies 23, 24 are conventionally at leastslightly rotatable and moveable longitudinally in the centralcylindrical bore 19. Where access to the perpendicular intersection ofthe bores 50, 51 with the axial bore 19 is prevented, as necessarilyoccurs in a radiation shield 11 such as that depicted in the drawing,precise alignment is difficult in the absence of special measures.

According to a further aspect of the scintillation counter describedherein, the photomultiplier tube assembies 23, 24 are so shaped as toinsure precise registry with the bores 50, SI.

Directing attention to FIGS. 8 through 12, the photomultiplier tubeassembly includes a scintillation crystal assembly which contains withina metallic housing 71 a cylindrical scintillator crystal 64, a pair ofterminal lighbtransmissive windows 72, 73, and a central bore,perpendicular to the scintillator crystal 64, which defines the samplereceiving zone 28 (FIG. 1]). The assembly 70 also includes a pair ofbosses 75, 76, respectively at the top and bottom surfaces, which aremachined so as to have flat horizontal upper and lower surfaces 77, 78and stop portions 79, 80. When the scintillator crystal assembly 70 isplaced within the axial bore 19 (FIGS. 3, 12) of the central segment 13,careful lowering of the sample conducting tube 58 to a predeterminedposition, and careful raising of the elevator tube 59, assures that theflat ends of the respective tubes will insure angular alignment of thescintillator crystal assembly 70 (and consequently the entirephotomultiplier tube assembly 23, 24), while abutment of the stopportions 79, 80 with the tubes 58, 59 (FIG. 3) insures longitudinalalignment of the scintillator crystal assembly 70. After adjustment, theassemblies 7 23, 24 are locked in position by tightening the gaskets100.

A highly advantageous scintillator crystal assembly is depicted in FIGSv8-H and includes the previouslydescribed cylindrical crystal 64 and theterminal windows 72, 73. Scintillator crystals made of such materi alsas the organic compounds anthracene, napthalene, chrysens, stilbene, orthe like, or inorganic materials such as calcium tungstate (scheelite)or the alkali ha'- lides, with optional dopants, are well known. Thecrys tal 64 is contained within a generally cylindrical aluminum can 81,and is provided with a gammatransmissive but light reflective alumina ormagnesia coating 82 around the cylindrical surfaces of the crystal 64cylinder and around the sample receiving zone or central cylindricalbore 28 (FIG. 8). The windows 72, 73 are of course light transmissive topermit the light scintillations to be observed by the photomultipliertubes 60 (FIGSv 3).

An improved elevator extension platform, which is best shown in FIGS.through 7, FIG. 11, and FIG. 12, is dthe detecting efficiency of thescintillation counter by providing a maximum aspect, or field of view,for the scintillator crystal assembly. If the photomultipliers were toobserve only the cylindrical sides of a sample container 55 (FIG. 11),gamma rays emitted from the top and the bottom of the container 55 wouldhave no access to the scintillator crystal. Accordingly, these rayscould not be counted, with the result that counting efficiency would berather low.

An additional feature is to have the minimum amount of absorbing mediabetween the radioactive sample being analyzed and the scintillatorcrystal, and also as close to 4 Pi counting geometry as possible, Inmany applications, the radioactive material is of very small volume,covering only the bottom of the test vial. The elevator stopping orcounting position is remotely adjusted to position the center ofradioactivity close to the center of the crystal. it can be readily seenthat for a sample of very small volume the optimum counting position iswith the elevator platform stopping at near midpoint. Also it is readilyseen that this elevator design allows more energy to strike thescintillating crystal.

As a partial remedy of this, the sample receiving zone 28 is madesubstantially longer than the effective vertical height of the sample55. Then, an elevator extension platform 90 (FlG. 5) is secured to theflexible elevator band or ribbon 52 so as to position the sample 55above the bottom ofthe sample receiving zone. The ex tension is providedby a plurality of thin vertical sample-supporting members 91 to permitgamma radiation emitted transversely from the bottom of the sample 55 toreach the scintillator crystal 64 (FIG. 11). The thin members 91 offerminimal interference with transmis sion of gamma ray photons and, ifmade of a stiff plastic such as polypropylene are capable alone, or incombination with a conical central portion 92, of supporting the sample55. The conical portion 92, as will be observed from FIG. 11, similarlyoffers little geometric interference to gamma rays. The bottom of theconical portion 92 terminates in a cylinder 93, having a web 94 which ispinned to the elevator band 52.

Thus it is apparent that there has been provided. according to theinvention, a scintillation counter that fully satisfies the objectivesrecited abovev We claim:

1. In a scintillation counter having a sample-receiving zone within amassive lead radiation shield,

at least one photomultiplier tube assembly responsive to lightscintillations produced from radiation emitted by said sample, said tubehaving an elongated shape and being removably contained within saidshield,

the improvement whereby said lead radiation shield is disassembleableinto a plurality of segments to allow installation and removal of saidphotomultiplier tube, and which segments, when assembled, preventstraightline access of external radiation through said lead shield intosaid sample-receiving zone or into said assembly, comprising:

a sample-zone-surrounding segment having an axial cylindrical boresufficiently large in diameter to receive one section of said elongatedphotomultiplier tube assembly,

an intermediate segment having an axial cylindrical bore coaxial withsaid sample-zone-surrounding segment for receiving a second section ofsaid photomultiplier tube assembly, said intermediate segment beinglongitudinally divided into two segments for radial outward movement ofeach seg ment to permit access to said second photomultiplier tubeassembly section, and

an end cap segment covering the bore of said intermediate segment, theparting surfaces of said segments thereby having no surfaces offeringstraightline access of external radiation through said lead shield intosaid sample-receiving zone or into said assembly.

2. Combination of claim 1 wherein said sample-zonesurrounding segmenthas a protruding collar on its parting face, and the intermediatesegment has a mating recess on its parting face.

3. Combination of claim 1 wherein said intermediate segment is dividedby a pair of parallel parting surfaces, each tangential to thecylindrical bore of said intermediate segment.

4. Combination of claim 3 wherein said parting surfaces are flat planes.

5. Combination of claim 1 in which said sample-zonesurrounding segmentincludes a base for supporting said segment at a stationary location,and said two intermediate segments each are mounted upon rollers forsupporting said segments for rolling movement in a radial direction withrespect to said sample-zonesurrounding segment.

1. In a scintillation counter having a sample-receiving zone within amassive lead radiation shield, at least one photomultiplier tubeassembly responsive to light scintillations produced from radiationemitted by said sample, said tube having an elongated shape and beingremovably contained within said shield, the improvement whereby saidlead radiation shield is disassembleable into a plurality of segments toallow installation and removal of said photomultiplier tube, and whichsegments, when assembled, prevent straight-line access of externalradiation through said lead shield into said samplereceiving zone orinto said assembly, comprising: a sample-zone-surrounding segment havingan axial cylindrical bore sufficiently large in diameter to receive onesection of said elongated photomultiplier tube assembly, an intermediatesegment having an axial cylindrical bore coaxial with saidsample-zone-surrounding segment for receiving a second section of saidphotomultiplier tube assembly, said intermediate segment beinglongitudinally divided into two segments for radial outward movement ofeach segment to permit access to said second photomultiplier tubeassembly section, and an end cap segment covering the bore of saidintermediate segment, the parting surfaces of said segments therebyhaving no surfaces offering straight-line access of external radiationthrough said lead shield into said sample-receiving zone or into saidassembly.
 2. Combination of claim 1 wherein said sample-zone-surroundingsegment has a protruding collar on its parting face, and theintermediate segment has a mating recess on its parting face. 3.Combination of claim 1 wherein said intermediate segment is divided by apair of parallel parting surfaces, each tangential to the cylindricalbore of said intermediate segment.
 4. Combination of claim 3 whereinsaid parting surfaces are flat planes.
 5. Combination of claim 1 inwhich said sample-zone-surrounding segment includes a base forsupporting said segment at a stationary location, and said twointermediate segments each are mounted upon rollers for supporting saidsegments for rolling movement in a radial direction with respect to saidsample-zone-surrounding segment.